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ESE370: Circuit-Level Modeling, Design,
and Optimization for Digital Systems
Lec 6: September 19, 2016
MOS Model
Penn ESE370 Fall2016 – Khanna
You are Here: Transistor Edition
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Previously: simple models (0 and 1st order)
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This week and next (4 lectures)
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Comfortable with basic functions and circuits
Detailed semiconductor discussion
MOSFET phenomenology
Don’t Blink!
Rest of term
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Implications of the MOS device
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Today
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MOS Structure
Basic Fabrication
Semiconductor Physics
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Metals, insulators
Silicon lattice
Band gaps
Doping
Field Effects
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MOS
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Metal Oxide Semiconductor
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MOS
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Metal – gate
Oxide – insulator separating gate from
semiconductor
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Ideally: no conduction from gate to semiconductor
Semiconductor – between source and drain
See why gate input is capacitive?
gate
drain
source
semiconductor
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Capacitor
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Charge distribution and field?
How much charge on plates?
gate
drain
source
semiconductor
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Idea
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Semiconductor – can behave as metal or insulator
Voltage on gate induces an electrical field
Induced field attracts (repels) charge in
semiconductor to form a channel
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Semiconductor can be switched between conducting and
not conducting
Hence “Field-Effect” Transistor
gate
drain
source
semiconductor
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Source/Drain Contacts
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Contacts: Conductors # metalic
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Connect to metal wires that connect transistors
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Fabrication
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Start with Silicon wafer
Dope
Grow Oxide (SiO2)
Deposit Metal
Mask/Etch to define
where features go
http://www.youtube.com/watch?v=35jWSQXku74
Time Code: 2:00-4:30
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Dimensions
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Channel Length (L)
Channel Width (W)
Oxide Thickness (Tox)
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Process named by minimum length
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22nm # L=22nm
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Semiconductor Physics
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Conduction
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Metal – conducts
Insulator – does not conduct
Semiconductor – can act as either
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Why does metal conduct?
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(periodic table)
http://chemistry.about.com/od/imagesclipartstructures/ig/Science-Pictures/Periodic-Table-of-the-Elements.htm
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Why does metal conduct?
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http://commons.wikimedia.org/wiki/File:Periodic_Table_of_Elements_showing_Electron_Shells.svg
Why does metal conduct?
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Conduction
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Electrons move
Must be able to “remove” electron from atom or
molecule
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Atomic States
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Quantized Energy Levels (bands)
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Valence and Conduction Bands
Must have enough energy to change level (state)
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Thermal Energy
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Except at absolute 0
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There is always free energy
Causes electrons to hop around
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….when there is enough energy to change states
Energy gap between states determines energy required
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Silicon Atom
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How many valence electrons?
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Silicon
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4 valence electrons
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Inner shells filled
Only outer shells contribute to chemical interactions
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Silicon-Silicon Bonding
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Can form covalent bonds with 4 other silicon atoms
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Silicon Lattice
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Forms into crystal lattice
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Silicon Ingot
1 impurity atom
per 10 billion
silicon atoms
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Silicon Lattice
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Cartoon two-dimensional view
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Outer Orbital?
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What happens to outer shell in Silicon lattice?
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Energy?
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What does this say about energy to move electron?
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Energy
Energy State View
Valance Band – all states filled
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Energy State View
Energy
Conduction Band– all states empty
Valance Band – all states filled
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Energy State View
Energy
Conduction Band– all states empty
Band Gap
Valance Band – all states filled
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Band Gap and Conduction
Insulator
Metal
Ec
8ev
Ec
Ev
OR
Ev
Ev
Ec
Semiconductor
1.1ev
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Ec
Ev
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Doping
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Add impurities to Silicon Lattice
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Replace a Si atom at a lattice site with another
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Doping
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Add impurities to Silicon Lattice
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Replace a Si atom at a lattice site with another
Add a Group 15 element
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E.g. P (Phosphorus)
How many valence electrons?
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Doping
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Add impurities to Silicon Lattice
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Replace a Si atom at a lattice site with another
Add a Group 15 element
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E.g. P (Phosphorus)
How many valence electrons?
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Doping with P
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Doping with P
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End up with extra electrons
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Donor electrons
Not tightly bound to atom
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Low energy to displace
Easy for these electrons
to move
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Doped Band Gaps
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Addition of donor electrons makes more metallic
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Easier to conduct
Semiconductor
0.045ev
1.1ev
Penn ESE370 Fall2016 – Khanna
Ec
Ev
ED
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Localized
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Donor electron is localized
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Won’t go far if no low energy states nearby
Increasing doping concentration
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Ratio of P atoms to Si atoms
Decreases energy to conduct
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Electron Conduction
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Electron Conduction
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Capacitor Charge
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Remember capacitor charge
++++++++
- - - - - - - - -
Penn ESE370 Fall2016 – Khanna
gate
drain
source
semiconductor
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MOS Field?
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What does “capacitor” field do to the Donor-doped
semiconductor channel?
Vgs=0
No field
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- - - -
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MOS Field?
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What does “capacitor” field do to the Donor-doped
semiconductor channel?
Vgs=0
No field
+
- -
- - - -
Penn ESE370 Fall2016 – Khanna
+ + +
- - -
Vcap>0
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MOS Field?
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What does “capacitor” field do to the Donor-doped
semiconductor channel?
Vgs=0
No field
+
- -
- - - -
=
Penn ESE370 Fall2016 – Khanna
+ + +
- - -
+++++
Vcap>0
Vgs>0
Conducts
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MOS Field Effect
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Charge on capacitor
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Attract or repel charges to form channel
Modulates conduction
Positive
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Attracts carriers
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Negative?
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Repel carriers
"  Disable conduction
+++++
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Doping
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What happens if we replace Si atoms with group 13
atom instead?
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E.g. B (Boron)
Valance band electrons?
B
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Doping with B
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End up with electron vacancies -- Holes
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Acceptor electron sites
Easy for electrons to shift into these sites
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Low energy to displace
Easy for the electrons to move
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Movement of an electron best viewed as movement of hole
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Doped Band Gaps
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Addition of acceptor sites makes more metallic
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Easier to conduct
Semiconductor
Ec
0.045ev 1.1ev
Penn ESE370 Fall2016 – Khanna
EA
Ev
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Hole Conduction
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Field Effect?
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Effect of positive field on Acceptor-doped Silicon?
Vgs=0
No field
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+ + +
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Field Effect?
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Effect of positive field on Acceptor-doped Silicon?
Vgs=0
No field
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+
+ + +
Penn ESE370 Fall2016 – Khanna
+ + +
- - -
Vcap>0
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Field Effect?
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Effect of positive field on Acceptor-doped Silicon?
Vgs=0
No field
+
+
+ + +
=
Penn ESE370 Fall2016 – Khanna
+ + +
- - -
+++++
Vcap>0
Vgs>0
No conduction
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Field Effect?
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Effect of negative field on Acceptor-doped Silicon?
Vgs=0
No field
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+
+ + +
Penn ESE370 Fall2016 – Khanna
- - + + +
Vcap<0
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Field Effect?
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Effect of negative field on Acceptor-doped Silicon?
Vgs=0
No field
+
+
+ + +
+ + +
- - -
=
Penn ESE370 Fall2016 – Khanna
- - Vcap<0
Vgs>0
Conduction
+++++
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MOSFETs
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Donor doping
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Excess electrons
Negative or N-type material
NFET
Acceptor doping
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Excess holes
Positive or P-type material
PFET
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Big Ideas: MOSFET
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Semiconductor can act like metal or insulator
Use electric field to modulate conduction state of
semiconductor
-----+++++
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Admin
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HW due Wednesday
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